Retroreflective articles are well known for applications such as highway signs, safety reflectors, and road markers. Generally, cube corner versions of these articles have a frontal lens of transparent, colored or uncolored resin, such as methyl methacrylate, polycarbonate, or vinyl, with a smooth front surface and a plurality of retroreflective cube corner elements on the reverse surface. The cube corner elements each have three reflecting faces.
Incident light from a remote source passes through the smooth front surface, reflects off each of the three faces of a cube corner element, and passes back through the front surface in a direction nominally 180° to the direction of incidence. In a perfect retroreflector, this light is returned to the light source in a direction exactly opposite to the incoming direction of light. Partially because of variations in the structure of a retroreflector, either accidental or by design, the reflected light is not returned only in a direction exactly opposite to the incoming direction, but rather is returned typically into a spreading pattern, centered on the exact return direction. This “imperfect” return reflection is still termed “retroreflection”. The spreading retroreflected light enables the retroreflector to be visible from directions slightly removed from the light source.
The angle between the incoming light source and the reflected light, and having a vertex at the retroreflector, is called the “divergence angle” and relates to the amount, in angular units, the retroreflected light diverges from perfect retroreflection. Conventional retroreflective articles are generally designed to be highly visible at long distances, corresponding to the “observation angle”, which is the angle between the incoming light source and the observer, having a vertex at the retroreflector.
In highway safety applications, such as highway signs and pavement markers, the retroreflector should reflect light from a vehicle's headlights back to the eyes of the driver of the vehicle. This is imperfect retroreflection, in which the observation angle, α, ranges between approximately 0° and more than 3°. The value of α in any given situation depends on the geometry of the vehicle and the driver and the distance from the vehicle to the retroreflective material. For example, the observation angle α for a large truck's right headlight and its driver at a distance of about 40 meters from a road sign will be approximately 3°, while the observation angle α for an automobile's left headlight and its driver at a distance of about 600 meters from a road sign should be approximately 0.05°.
Also associated with the observation angle, α, is a rotation angle, ε, which is a measure of the direction of the divergence, also known as the azimuth angle. The value of ε will be different for left and right headlights of a vehicle, and will also depend on the vehicle and driver geometry and the position of the road sign. For sheeting that will be mounted in random orientation on road signs, retroreflection is required at every value of ε. The angles α and ε are defined in ASTM E808, Standard Practice for Describing Retroreflection, which document refers to divergence angle as “observation angle”, α.
Ideally, retroreflective sheeting used in road signs will produce a pattern of retroreflected light having sufficient intensity over a range of observation angle α values and rotation angle ε values. However, various retroreflective articles are sensitive to the orientation of the article to the observer. That is, depending on the rotation angle ε, the observer will experience various intensities of retroreflected light. From one particular rotation angle ε, the retroreflection may be relatively intense, while at another rotation angle ε, the retroreflection may be relatively weak.
This sensitivity to orientation of an observer in relation to a retroreflective surface at various rotation angles ε, can be addressed in at least two ways. One way is to form a retroreflective article by using a mold made by “pinning,” wherein a cluster of metal pins are assembled, each pin having a single cube corner machined and polished on one end. The pins would typically have a triangular, hexagonal, square, or rectangular cross-section. The pins could then be bundled together so that their machined tips could be used to form an array of “male” cube corners, and such a bundle would be used as a master to electroform a “female” mold. The mold would then be used to form an array of male prism elements in glass or plastic. It is well known that variations in the size or shape of the faces of the machined pin ends, or in the angles between the faces (dihedral angles), or in the flatness of the faces or the flatness of the front surface of the formed retroreflector, can all change the pattern of retroreflection and thereby determine the regions around the light source in which the retroreflection is visible.
Pinning allows for flexibility in the manufacture and design of cube corner arrays. Various shapes can be utilized, other than equilateral triangle cube corners, and each pin could be individually tailored in its geometry and orientation to contribute to the aggregate performance of the array.
However, because of manufacturing limitations, the pin typically has a geometric shape on the end of the pin that is about 0.040 inches (1 mm) square or larger (hereinafter called “macrocubes”). Hexagonal pins typically may have a dimension across parallel flats on the order of about 0.10 inch (2.5 mm). Rectangular pins have a short dimension of about 0.070 inch (1.8 mm) and a long dimension of about 0.120 inch (3.0 mm). Macrocubes, because of their height, are too large for use in the manufacture of thin flexible retroreflective sheeting requiring smaller retroreflective prism elements.
In order to make arrays of smaller prism elements (hereinafter, “microcubes”), a different technique has typically been used. In this technique, microcubes can first be formed in a master substrate. One method of forming the microcubes is by direct machining or ruling, wherein parallel rows of V-shaped grooves are cut into a substrate to create a pattern of grooves which intersect to form cube corner elements. Three such sets of V-shaped grooves can form an array of triangular-shaped cube corners. Arrays of such microcubes can be used as a mold for plastic retroreflective sheeting. The machining method often employs diamond cutting. By “diamond cutting” it is meant ultra precise direct mechanical machining of precision elements using a diamond cutting tool comprising a machining tool (e.g., lathes, turn-mills, rotary transfers, or non-rotary type free-form generation tools such as raster mills) and a diamond cutting element (such as a point, blade or edge) that scores, cuts, grinds, gouges, grooves, or otherwise modifies a surface by bringing the diamond cutting element into contact with the surface to be modified. The diamond cutting tools are used for engaging in on-axis or off-axis turning, ruling, fly-cutting, or micro-prismatic cutting operations and can produce sub-nanometer level surface finishes (peak to valley distance of the surfaces formed by diamond cutting) and sub-micron form accuracies. Diamond cutting machines often are computer numerical control (CNC) machine tools utilizing electric motors and piezoelectric actuators used for accuracy. The grooves produced by diamond cutting have smooth edges that are substantially free of burrs or other imperfections of micron size or greater that are associated with conventional cutting or machining techniques.
Another method of forming microcubes in a substrate involves forming rows of microcube corner prisms on the edge of thin plates or laminae. This technique, while being more difficult than direct machining methods, has the advantage of providing more freedom for different cube shapes and individual tailoring. These plates can be stacked together to form an array of prism elements.
A master of “male” or “female” cube corner elements can be used to make a sequence of replicas, copies or “tiles”, of alternating gender (i.e. first generation and second generation), such as by electroforming. For example, if the master has protruding (“male”) prism elements, then the first generation copies of the master will have recessed (“female”) prism elements, i.e., the tiles will have opposite configuration from the master. The second generation copies will be substantially identical to the master, that is, the tiles will have protruding prism elements. At any stage, the first or second generation copies of the master can be diced or cut into a desired shape, and the diced tiles are then assembled together to form a tiled article. Assembling tiles together into a larger assembly is herein known as “tiling” and larger assembled arrays are known herein as “tiled articles,” wherein several tiles having smaller arrays of prism elements are joined into a larger tiled article. In turn, the tiled articles can also be copied to form a larger tile, and then joined together to make further, even larger, tiled articles. When referring to a “tile”, it is meant a unitary or single-piece structure or substrate. When referring to a “tiled article”, it is meant a multi-piece structure, formed by joining two or more tiles or substrates together. When referring to “master”, it is meant any structure that is used to form replicas. That is, a master can be a single-piece substrate or a tiled article. In either event, the replica of the master or a diced portion of said replica can be considered to be a tile, i.e. a unitary, single-piece structure. For example, if either a single-piece structure or a multi-piece structure (i.e. a tiled article) is duplicated such as by electroforming, the replica or a diced portion thereof can be considered to be a tile (i.e. single-piece structure) because electroforming will produce a replica having a unitary, single-piece structure without seams.
After a series of copying and tiling stages, a single “mold” can be formed. The “mold” can be used to make production tools, such as by electroforming, which tools can be used to form microcube or other retroreflective elements on an expanse of plastic sheeting material such as by embossing, casting, compression molding or other methods known in the art. Alternatively, the mold itself can be used to make a retroreflective article.
In either the direct machining or laminae methods of forming micro prism elements, such prism element arrays will typically only have one or two prism orientations present, and thus a high sensitivity to orientation. So it is desirable for copies of such assemblies to be diced as described above to form tiles. Orientation sensitivity is addressed by alternating the orientation of the individual tiles in the tiled article. In particular, prism orientations of the individual tiles are varied between adjacent tiles. This alternating or varied prism orientation between individual adjacent tiles produces a retroreflective article that is less sensitive to the rotation angle ε, than would an article having only a single prism orientation. This is done to provide larger aggregate cube corner arrays with reduced sensitivity to orientation. Further, the size of the tiles can be reduced to make the visual contrast between differently oriented tiles less noticeable, i.e. less resolvable to the human eye.
However, this tiling technique has at least two limitations, among many. The process of tiling arrays of microcubes encounters similar limitations to those seen in the pinning of macrocubes and some others limitations. First, very small tiles are difficult to physically handle or maneuver. This difficulty is a deterrent to making smaller sizes of individual prism arrays in a tiled article. The smaller the tile, the more difficult it becomes to handle and position. This adds to production cost and time for the tiled molds. Second, typical dicing processes used to make tiles of various shapes and sizes, produce crude edges of degraded performance on the tiles. That is, conventional dicing produces tiles that can have an edge with partial and/or damaged prism elements and tiles with edges that are not sufficiently straight. As the tile size is reduced, the proportion increases between that of the degraded edge portion and the rest of the prism array, thus reducing the fill factor for retroreflective articles made with such tiled molds. Further, if the tiles are not accurately sized or shaped and have crude edges, an assembly of such tiles edge-to-edge may have small gaps between the tiles, i.e. wide seams between tiles, which can create problems during replication and similarly degrade the retroreflective fill factor of the articles produced from such molds.